1. Field of the Invention
The present invention relates to an apparatus and method for housing a lens and a photodetector, the photodetector being cooled to a desired temperature.
2. Description of the Related Art
Various photodetectors have been developed that are capable of detecting very faint light. Such detectors may be in the form of, for example, a photomultiplier tube (PMT), a photocathode coupled to a multichannel plate (MCP), an avalanche photodiode (APD), a charged couple device (CCD) etc. Notably, in applications where accurate detection of very faint light is required, such detectors need to be cooled to a specified temperature. An example of an APD used in cooled mode to detect light emission from semiconductor devices is described in U.S. Pat. No. 6,720,588, which is commonly assigned to the present assignee, and which is incorporated herein by reference in its entirety.
One application where detection of very faint light requires sensitive detectors that operate in a cool mode is for testers used for testing and debugging semiconductor circuits. One such system that utilizes an APD described in the above-cited patents is described in U.S. Pat. No. 6,621,275, commonly assigned to the current assignee and incorporated herein by reference in its entirety. Such a system is commercially available under the trademark EmiScope® from assignee, Optonics Inc., a Credence Company, of Mountain View, Calif. In this system the APD is required to be cooled to well below zero degrees Celsius.
While there are various methods for cooling semiconductor devices, such as photodetectors, one particularly useful method is using the Peltier effect. The Peltier effect and cooling apparatus and methods using the Peltier effect are described in, for example, U.S. Pat. Nos. 6,477,844; 6,125,635; and 6,109,039, which are incorporated herein by reference in their entirety.
Another problem described in the literature is protecting the detector from deterioration due to moisture, hydrocarbons and other gas contaminants. These effects cause the detector's performance to deteriorate over time. Therefore, various works have proposed enclosing the detector in a vacuum enclosure having a window through which photodetection can be performed. See, for example, Vacuum Sealing Adds Life to Scientific CCDs, Dr. Hugh Cormican, Photonics Spectra, April 2004.
FIG. 1 depicts an arrangement of a conventional housing for a cooled photodetector. In the arrangement of FIG. 1, the cover 10 is affixed to a base 60 via sealing mechanism 50. The cover includes a window 40 that is aligned with photodetector 20. The photodetector 20 is held against a thermo-electrical cooler (TE cooler) 30, which, in this case, is shown to comprise of three stages of Peltier effect layers. The TE cooler 30 collects heat from the detector 20 and transfers the heat to the base 60, which is also used as a heat sink. The heat from the base 60 is then removed, as illustrated by the large arrow. The enclosure is evacuated and may be filled with inactive gas, such as argon. This entire arrangement is then aligned with the lens holder 70, which houses lens 80.
One drawback of prior art (as is clear from FIG. 1) is the fact that the focusing lens is constrained to be far from the active area of the photodetector, because of the necessary window in between. For that reason, it is very hard to achieve small enough light spot sizes with available aspheric lenses, when the lens working distance must be longer than 6 mm (at best). Another drawback described in the literature is that the window may collect moisture and degrade the detection efficiency. Also, any coating, such as antireflective coating provided on the window may also degrade with time. Accordingly, a better solution is needed that would enable both cooling and protecting the light sensor, provide a small light spot on the detector, and, at the same time, avoid the drawbacks of the prior art.